The present invention relates to a method for constructing a tunnel or underground facility which has large sized cavities therethrough. More particularly, the present invention relates to a method for constructing a rectangularly configurated tunnel wherein the roof of the tunnel is preliminarily and artificially constructed in the earth, and thereafter the rectangular tunnel of large size is strongly built up.
Despite remarkable progress which has been made in tunnelling techniques, the configuration of most tunnels has remained uncharged. that is, tunnels still have a curved cross section, such as semi-circular or semi-oval sections.
However, it becomes difficult, with the above-mentioned type of tunnel, to excavate the tunnel if the tunnel has a floor width of more than about fifteen meters. This is because of limitations in tunnel height. Further, the enlargement of the tunnel necessitates that further excavation be carried out with a strong shell formed therearound, whereas it is difficult to have a semi-circular or semi-oval shell formed without loosening ground rock around the tunnel. Additionally it is necessary in a tunnel of curved cross section to enlarge the upper or facing area of the tunnel to an extent more than is required for the vehicles to pass through the tunnel. This means the increase of useless and unnecessary tunnel area, which further adds to the amount of excavation necessitated.
In view of the foregoing, a tunnel of large sectional area has not been made with known tunnelling techniques and, instead, a plurality of tunnels are excavated when necessary to fulfill the requirement caused by heavy traffic.
But with recent traffic patterns, the width of the traffic zone is required to be broadened into a double track or driveways having several lanes, and it is desired to have the track running closely parallel to the driveways. This requirement in turn results the serious problem of establishing a reliable technique for forming a tunnel of the necessary width. However, because as mentioned above a tunnel having a curved cross section is confined to have a narrow traffic width, it has been practically impossible to excavate a single tunnel having a width of 20 to 30 meters despite the need therefore.